iftop does for network usage what top(1) does for CPU usage. It listens to network traffic on a named interface and displays a table of current bandwidth usage by pairs of hosts.

Overview of IFTOP
* iftop (interface top) derives the name from the standard unix top command. top command displays real-time CPU Usage. iftop command displays real-time network bandwidth usage.
* iftop displays the network usage of a specific interface on the host.
* Using iftop you can identify which host is responsible for slowing down your network.
* To find out which process is causing the problem, note down the port number from the iftop and use netstat -p to identify the process.
* iftop monitors your network activity, and displays a table of current bandwidth.

Using:
Go to your cosole and use command: iftop to start monitoring the bandwidth usage.
you can also specify a particular interface with the -i option: iftop -i eth1, some other options ..
* -p Enables promiscuous mode, so the traffic on any interface (if there is more than once) is checked and counted

* -P Shows also the port that connection is using both on both side

* -N Do not resolve port names, which is the default behavior when you enable the -P option, so it will shows you :www or :80